Why a fish tank?! It has no ventilation and the glass will get (and stay) too cold or too hot. It's not what he lives in, right? Get yourself a decent pet carrier and line it with a generous layer of paper pulp bedding and lots of hay. Whenever you stop for gas/bathroom, hang his water bottle on it so he can stay hydrated. Don't leave the water bottle hanging on it while you drive though or it will leak and wet all the bedding and hay. You could provide moisture rich fruits or veggies at intervals. Watermelon is a good one for travel. Also red bell pepper. Take some pieces in a baggy. He should be getting daily fresh veggies as a regular part of his diet.

Make sure he is not left in a hot car for any length of time. He can stand a little cold if he has lots of bedding and hay in the carrier with him. If it is really cold, take a blanket to throw over his carrier when you open/close the doors.

He will be fine traveling as long as you have a larger cage for him to be put into once you reach your destination. He will not live long in a glass aquarium. C&C grids are great for making a temporary cage (use a tarp or shower curtain for a floor) and don't take up much room in a car.

I've personally only traveled 6 hours. I used a carrier with ventilation and paper bedding (instead of the usual fleece). Food and hay in bowls. Attaching the water bottle to the cage is the biggest mistake I made. Like they said, give him wet veggies and fruits (I used watermelon, cucumber, red leaf lettuce, green peppers) to keep hydrated between stops (when you should offer the water bottle). Never leave him in the car for long, obviously. I think making him a cuddle sack is a great idea! That will give him a place to feel safe. Try to introduce it to him before the drive so he can his smells on it, that might help even more.

My husband and I actually just took both of our piggies on a 13.5H drive this summer, AND one of my pigs had just been diagnosed with heart problems, so it was extra important that we kept them comfortable. To be honest, I would not have been able to do it alone, but here's what we did:

We bought 2 hard plastic cat carriers with lots of ventilation and filled them up with a thick layer of paper bedding and lots of hay. We secured them SUPER tightly--seatbelted in, completely packed in by things like bedding, pillows, etc. from all angles. We also had a small cooler with lots of green pepper and cucumber, cold water, and pre-mixed critical care. Every 1-2 hours we would offer them fresh water with a 10mL feeding syringe and give them wet veggies/critical care. We tried to clean out a little of the cage and add more bedding twice during the trip to keep them clean, and added more hay whenever we thought it looked a little sparse.

We only stopped twice for ourselves to get food and gas--this is the thing that I think will be most difficult about your situation, because I would NOT leave a guinea pig alone in the car for very long. Obviously you have to get up and take care of yourself! But I would recommend either bringing a lot of food for yourself, or making sure you plan on drive-through places to minimize the amount of time spend away from the car. I once left my poor husband sitting with them for 30min waiting on my lunch in a rest stop!

I would not use just towels for two reasons: one, it will get dirty quickly, and two, they can bury underneath it and they may overheat if you have the heat on in the car. Be careful about where the airflow is going too--you don't want it to be directly blowing on your guinea pig's cage.